Mutual Monomer Orientation To Bias the Supramolecular Polymerization of [6]Helicenes and the Resulting Circularly Polarized Light and Spin Filtering Properties

We report on the synthesis and self-assembly of 2,15- and 4,13-disubstituted carbo[6]helicenes 1 and 2 bearing 3,4,5-tridodecyloxybenzamide groups. The self-assembly of these [6]helicenes is strongly influenced by the substitution pattern in the helicene core that affects the mutual orientation of the monomeric units in the aggregated form. Thus, the 2,15-substituted derivative 1 undergoes an isodesmic supramolecular polymerization forming globular nanoparticles that maintain circularly polarized light (CPL) with glum values as high as 2 × 10–2. Unlike carbo[6]helicene 1, the 4,13-substituted derivative 2 follows a cooperative mechanism generating helical one-dimensional fibers. As a result of this helical organization, [6]helicene 2 exhibits a unique modification in its ECD spectral pattern showing sign inversion at low energy, accompanied by a sign change of the CPL with glum values of 1.2 × 10–3, thus unveiling an example of CPL inversion upon supramolecular polymerization. These helical supramolecular structures with high chiroptical activity, when deposited on conductive surfaces, revealed highly efficient electron-spin filtering abilities, with electron spin polarizations up to 80% for 1 and 60% for 2, as measured by magnetic conducting atomic force microscopy.


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Optical Rotation measurements were performed on a Jasco P-2000 polarimeter.
The thermodynamic parameters associated to the supramolecular polymerization mechanisms were derived by applying the solvent denaturation model published by Meijer and coworkers. S1 Fluorescence spectra were recorded on a FL 920 Edinburgh fluorimeter. Fluorescence quantum yields Φ were measured in diluted solution using the following equation: Where the subscripts ST and X denote standard and sample respectively, Φ is the fluorescence quantum yield, Grad is the gradient from the plot of integrated fluorescence intensity vs absorbance, and η the refractive index of the solvent. Reference for fluorescence quantum yields used herein are quinine sulfate in 0.5 M sulfuric acid (Excitation of reference and sample compounds was performed at the same wavelength).
The decays were calculated by using an Ocean optics QEPro CCD detector (Range: 350-1100 nm). The excitation source has been a picosecond laser diode (10 KHz to 100 MHz) at 375 nm capable of operating in Burst mode below 10 KHz. For the detection windows, signals in UV-Vis (350-950 nm) are recorded as a function of time over a range of 135 nm simultaneously using a Hamamatsu C10910-25 streak camera mounted with a slow single sweep drawer. The range of measurable lifetimes is 100-300 µs.

General Protocol for Sonogashira Coupling
The corresponding bis-ethynyl-[6]helicene and aryl iodide were dissolved in a mixture of dry THF and Et3N (1/1) and was degassed by 3 Vacuum/Ar cycles. CuI and Pd(PPh3)4 were added and the reaction mixture was stirred for 2 hours at 50ºC. The organic layer was evaporated under reduced pressure and the residue was purified by column chromatography (silica gel) using heptane/ethyl acetate (1/1) for the ethylene diamide derivatives 1 and 2. The fractions containing the product were evaporated under reduced pressure and the residue was purified by recycling GPC eluting with CHCl3, affording the corresponding the pure compounds. S-4

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Additional AFM Experiments AFM experiments were carried out to visualize the morphology of the supramolecular assemblies generated by 1 and 2. Sample was prepared by spin-coating 10 M solution of the corresponding assembly onto freshly cleavage of HPOG. The results are summarized below.

Sample preparation for magnetic-conductive atomic force microscopy (mc-AFM):
The substrates for the mc-AFM studies were prepared using the thermal evaporation deposition technique. A 100 nm layer of Ni layer is sputtered, followed by a 10 nm layer of Au layer on a Si wafer, with a 10 nm Ti layer as an adhesion layer. The deposited multilayer surfaces were cleaned by immersing them first in boiling acetone and then in ethanol for 10 minutes. The surfaces were also cleaned with UVozone for 15 minutes, followed by a final 45-minute incubation in warm ethanol. The role of the Ni/Au surfaces is to allow spin polarization of electrons injected from the surface into the chiral molecules induced by a magnetic field. The molecules were deposited by drop-casting the solutions onto the cleaned surface.

Measurement using mc-AFM:
Magnetic field-dependent current-voltage (I-V) characteristics of the prepared samples were determined using a multimodal scanning magnetic probe microscopy (SPM) system equipped with a Beetle Ambient AFM and an electromagnet with R9 electronic controller (RHK Technology).
Voltage spectroscopy for the I-V measurements was performed by applying voltage ramps with a non-magnetic Pt tip (DPE-XSC11, μmasch with spring constant 3-5 Nm -1 ) in contact mode.  Figure S17 and S18, respectively. S-20

Magnetoresistance measurements
A Crossbar configuration was used for the magnetoresistance (MR) device that was produced as described in ref. 11a (Figure 8a). Molecules were spin coated on the top of bottom electrode. On the top of polymer film an insulating buffer layers of 1.5 nm magnesium oxide (MgO) were grown by e-beam evaporation followed by Ni and Au having a thickness of 40nm and 20nm, respectively, using a shadow mask with a line width of ~20μm. The device was subsequently attached to a cryogenic chip carrier and electrically connected by wire bonder (Au wire). The sample was measured by 2T-cryogenics system (Cryogenics Ltd) with different temperature. A magnetic field of up to 1T was applied along the direction of current i.e. perpendicular to the sample plane. The resistance of the device was measured using standard four-probe method. DC current of 0.1 mA was applied using a Keithley current source (Model 2400) and the voltage across the junction was measured using a Keithley nanovoltmeter (Model 2182A).

Solid State ECD and UV-Vis studies
The samples were dissolved in MCH at concentrations around 10 M solution and deposited onto quartz subsrates. The UV-Vis and ECD spectra were measured at different rotating angles and from front and back sides resulting in average spectra.